1. Technical Field
The present invention generally relates to Low Drop Out (LDO) voltage regulators.
It finds applications, in particular, in batteries or any equipment that needs constant and stable voltage supply such as mobile phones, cordless extension phones, MP3 players.
2. Related Art
Voltage regulators are usually used to provide a stable power supply voltage independent of load impedance, input-voltage variations, temperature and time. An output capacitor is generally used to stabilize the voltage regulator and to filter perturbations due to load transients.
In some cases, output capacitors have to be changed to allow a better flexibility. For example, if the output load is quiet, a small capacitance (for example 100 nanofarads nF) can be used. On the contrary, if the output load is noisy, a high capacitance (for example 2.2 microfarads uF) would be more adapted.
However, while changing the output capacitor, the system comprising the voltage regulator and the output capacitor can become unstable with a risk of degrading the output component. Indeed, in prior art systems, each couple of output current and output capacitor needs a dedicated LDO driver for stability reasons and it is not possible to change “on demand” the value of the output capacitor.
Referring to FIG. 1, there is shown a LDO regulator according to the prior art. The LDO regulator comprises a differential stage 101, a buffer stage 102, a PMOS transistor 103, an output impedance 104, which is supplied with an output voltage VOUT, and a feedback circuit 105. Based on the output voltage VOUT, the feedback circuit 105 is adapted to generate a feedback voltage VF1 to be transmitted to the differential stage 101. The differential stage 101 is adapted to generate an intermediate signal that is forwarded to the buffer stage 102, which has a unity gain, based on the feedback voltage VF1 and based on a reference voltage VREF1. The buffer stage 102 controls the gate of the PMOS transistor 103, which source is supplied by a high positive supply voltage VDD and which drain is connected to the output of the LDO regulator. The output impedance ZOUT 104, which is at the output of the LDO regulator, can be constituted of an output capacitor in parallel with a circuit, which is supplied by the regulator with an output voltage VOUT. Depending on the output load, it can be advantageous to change the output capacitor.
Referring to FIG. 2, there is shown a Bode diagram in open loop in a system of the prior art, such as the LDO regulator shown on FIG. 1, representing phase and gain in decibels (dB) versus frequency in Hertz (Hz), for different values of output capacitors. Curves 1.1, 1.2, 1.3 and 1.4 respectively represent phase versus frequency with an AC test signal at an inverting input VF (which will be further detailed referring to FIG. 2) and curves 2.1, 2.2, 2.3 and 2.4 respectively represent gain versus frequency, for the following capacitance values of output capacitors: 2.2 uF; 1 uF; 0.47 uF and 100 nF.
As it can be observed on FIG. 2, when the capacitance value of the output capacitor decreases, frequency of the dominant pole (in the gain curves) 4.1, 4.2, 4.3 and 4.4 is shifted toward higher frequencies. The phase margins, meaning the phase values observed when the gain is equal to zero, are correspondingly reduced. For example, for an output capacitor value equal to 0.47 uF, phase margin 3.3 is approximately equal to 33 degrees, which is stable but low (meaning that transient response is not enough damped), whereas for an output capacitor value equal to 100 nF, phase margin 3.1 is around zero. It is noted that in order to remain stable, phase margins should be held around 45 degrees. Thus, decreasing the output capacitor value from 0.47 uF to 100 nF means that the system of the prior art becomes really unstable.
Other factors contribute to instability of the system, such as the current consumption of the LDO voltage regulator for example.
Thus there is a need to design a LDO regulator that remains robust and stable while an internal or external factor is varying (such as an output capacitance value or the current consumption).